1. Field of the Invention
The present invention relates to a luminance and chrominance (Y/C) signal separator.
2. Description of the Prior Art
As is generally known, in order to separate luminance and chrominance signals from a composite video signal, a comb filter utilizing a frequency interleaved relationship between the two signals has been used. In such a comb filter, correlation is maintained in relation to a signal delayed by a horizontal scanning line period resulting in undesirable color smear appearing in the vertical direction of a screen.
There is also known a luminance and chrominance signal separator which does not produce color smear and referred to as a dynamic comb filter or a logical filter. For example, in Japanese Patent Laid-Open No. 61-23492, a technique is disclosed wherein a color video signal is supplied to a vertical correlator to separate a luminance signal therefrom, and a chrominance signal is separated by a filter and a horizontal correlator arranged to receive an output from the vertical correlator. In addition, a Y/C separator using a non-linear vertical comb filter and a horizontal correlator, which are employed in the present invention, is disclosed in Japanese Patent Laid-Open No. 64-29187. This separator includes a vertical correlator, which samples three points vertically aligned on a screen so as to determine vertical correlation between the lines containing the three points, and then separates the chrominance signal.
In general, non-linear processing is performed in a separator employing a correlator. Therefore, when digital processing using a sampling frequency three to four times a subcarrier frequency is performed, harmonic distortion due to foldover occurs in the video band.
In particular, when a zone plate test chart serving as a test pattern for allowing visual observation of two-dimensional frequency characteristics of a processing system, or a sweep test chart in which a horizontal width of an image is continuously changed in a vertical direction is displayed, interference fringes (moire) due to foldover appear.
In order to eliminate the above interference fringes, the sampling rate may be increased so as to perform an over-sampling processing. Therefore, even if foldover noise is generated, the noise may fall outside the video band. However, this method might not be practical because it requires a large-scale digital circuit operating at a very high speed.